I remember Tyll Hertsens did several measurements, repositioning the headphones between each in order to find an average in the treble, which typically changes drastically depending on the position, where the spikes and sharp dips "move" because the resonances and cancelings changes.
Yes Tyll did average multiple measurements, and that was one of the reasons I liked his approach better than most.
I assume ... Tyll probably explained, but I don't recall precisely ... that many of the spikes & dips & resonances & cancelings are largely produced by the measuring gear & set-up. Hence my use of the term "artifacts." What that tells us is that for the measuring gear, the placement of the headphones on the dummy's head can have a significant effect.
What is unclear ... to me, and I confess I've never bothered to look for research on this ... is whether the placement of headphones on a human's ear is equally significant. In general, I don't think it is. We frequently read things like "we all hear differently, everybody's inner ear is shaped differently" etc. But it's rare that I perceive much difference if I shift the position of a specific model of headphones around my ear.
Better evidence is provided if we listen w/o headphones to a sound signal. Like a mono speaker. If I am facing directly at the speaker, I hear a certain tonal range etc. If I angle my face 5 degrees left, I don't perceive any difference. 10, 20, 40, 70 degrees, left, right, up, down ... very little difference in what I am hearing. When I rotate my head in various directions while listening, I do not hear any bumps/dips/irregularities/discontinuities at certain orientations. And yet there can be no question that the sound waves are arriving from different directions when I do that, and the patterns of resonances & cancelings inside the outer and inner ear therefore MUST be different.
If that must be happening, why can't I perceive it?
One (partial) answer could be that I am an untrained and therefore poor listener with lousy discrimination. Guilty as charged. Probably true for many of us. (Some day I will bore you with my theory about why being an untrained listener is a good strategy for the thrifty audiophile.)
My more general theory is it's because we have a two-part hearing systems. Our ears are the external sensors that monitor our local environment for soundwaves. Each sensor has its technical limits, which generally degrade with age. But there is a semi-magical processing unit also involved, called the brain. The autonomous part of each brain learns to synthesize its sensory inputs and produce what I'll call "the angle-adjusted (normalized) sound spectrum", which it then passes onto the evaluative part of the brain. This autonomous processing will drop stuff, fill in holes, do smoothing, who knows .. it's magic.
And so per my theory, part of the reason why when we listen to headphones they don't sound spikey & dippy like the charts is because our brains are applying advanced filtering and smoothing. Including the brain's expectation (memory) of what the sound SHOULD sound like. Trained listeners can overcome some of that, but most of us can't/don't detect relatively small effects.
A corollary of this theory helps explain why "everybody's inner ear is different" is technically true but largely irrelevant for most people, i.e., those with "normal hearing." My brain tells me "normal sound spectrum, or not normal sound spectrum", given my body's microphones and my brain's lifetime of synthesizing/normalizing those inputs. Your brain is telling you "normal sound spectrum, or not normal sound spectrum", given your body's microphones and your brain's lifetime of synthesizing/normalizing those inputs. The two brains are generally going to arrive at the same interpretations.
In conclusion ... I'm going to pretend there was some point to this speculative ramble ...
another reason why h/p frequency charts aren't generally helpful is because humans don't process sounds like machines do, and vice versa.